Early computers used only discrete (individual) electronic components connected into circuit boards to form computer circuits. Circuit boards may be formed from epoxy cloth filled with fiberglass and partially cured to produce material known throughout the art as prepregs which are mechanically drilled to provide holes through the boards. The board surface and holes may be plated with a thin coating of copper metal (Cu) which is selectively coated with a material resistant to etching; the exposed Cu is etched away while the portion that is protected by the resist forms a wiring pattern over each major surface of the board and form plated through holes (PTHs) extending through the board to interconnect the two wiring layers. In early computers, each diode, transistor, resistor, and capacitor was individually positioned on the circuit board with leads inserted through the PTHs and then soldered in place to form an electronic circuit.
Transistors are devices which control the flow of electricity (current) between two electrical connections. The flow is controlled by regulating the potential (or current depending on the type of device) applied to other connections of the transistor. Generally, wires (called leads) are attached to each connection of the transistor to conduct electrical current or to apply the potential (or current) to the connection. Typically, two or more wires are connected to conduct current through the transistor and one or two wires are connected to control the electrical current through the device.
Transistors are devices manufactured in a highly pure crystal of semiconductive metal (e.g. Silicon, Geranium, Gallium-Arsenide) by multiple steps in which various other metals (e.g. Arsenic, Phosphorus and Boron) are diffused into the crystal at different selected surface locations. In one method called evaporation, the other metals (called impurities or dopants) are diffused into the silicon (Si) by positioning or forming a mask over the Si surface, placing the Si substrate in a very hot vacuum chamber, and heating other metals to their boiling point to provide a metal gas or vapor of the other metals in the chamber. The mask has openings through which the evaporated metal vapors reach the surface of the Si metal and atoms of the other metals slowly move into the Si metal.
In the late 1950's, integrated circuits comprising multiple transistors in a single crystal substrate of Si were developed. To manufacture these integrated circuits, a layer of Silicon dioxide (SiO.sub.2) is formed over the surface of the crystal to cover the devices, and cavities are selectively etched through the SiO.sub.2 to access the connections of the devices. The devices are interconnected by forming a pattern of metal wires over the SiO.sub.2 surface and in the cavities. The wires are produced by forming a mask or positioning a mask over the SiO.sub.2 surface in a vacuum chamber and providing a vapor of a metal such as Aluminum (Al) which does not tend to diffuse into the Si crystal and which condenses on the surface at openings in the mask to form wires and form conductive vias in the cavities through the SiO.sub.2. Additional wiring layers may be provided by forming dielectric layers with interconnection cavities and depositing additional wiring layers over respective dielectric layers. The completed wiring is covered by a layer of dielectric passivation to protect the wiring layers and Si devices. A multitude of openings in the passivation at respective contacts of the wiring layers allow the integrated circuit to be connected to leads which are connected into a circuit board.
More recently, in order to increase production speeds, sputtering and ion implantation methods have largely replaced metal evaporation for diffusing impurities into Si. Integrated circuits may include as many as 5 wiring layers separated by dielectric layers. The first wiring layer is usually poly-silicon (p-Si) and the subsequently formed wiring layers are usually Al deposited by sputtering or evaporation. Since the 1950's the density of devices and wiring in integrated circuits has vastly increased. Today, millions of transistors fit in a square centimeter, and hundreds of deposited wires can fit through a space no wider than an average human hair. The number of connections has increased from the 3 or 4 leads typical for discrete transistors to more than 500 ball type surface-to-surface connections for some integrated circuits.
Due to the high density of interconnections and to protect the computer chip from environmental hazards, integrated circuit chips are usually packaged in a component containing a very fine line circuitized substrate and leads or terminals to connect the substrate to the circuit board. Contacts on the chip are connected with contacts on the substrate of the package by either wire bonding or flip-chip connection. In wire bonding, very fine wires (1 to 3 mil) are connected between corresponding contacts of the chip and package substrate. In flip-chip connection, an array of bumps on the surface of a flip-chip are soldered to a corresponding array of pads on the package substrate. The most common integrated circuit components are dual in-line packages (DIPs) and single in-line packages (SIPs). These are flat rectangular packages with multiple leads (pins) extending from one or both longer edges of the package, and the leads are spaced to fit into one or two rows of PTHs of the circuit board. Another common type of pin-in-hole component is the pin grid array (PGA) in which pins extend from a major surface of a ceramic or organic substrate into a matrix of PTHs on a circuit board. The pins of such pin-in-hole packages may be connected to the PTHs by dipping the board into a wave of molten solder.
More recently components have been connected onto an array of pads on a surface of the circuit board in a process called surface mounting. Common types of surface mount components include quad flat packs (QFPs) and thin small outline packages (TSOPs). These packages include a thin, square or rectangular package substrate with leads extending outward from each edge of the package substrate and down onto the array of pads on the circuit board which leads have a J or gull-wing shape.
Other newer types of surface mount components include ball grid array (BGA) modules and column grid array (CGA) modules in which a matrix of balls or columns extend from one side of the package for surface-to-surface connection with a corresponding matrix of pads on the circuit board surface. The package substrates of BGA and CGA modules may include a tape of flexible layers of polyimid and patterned Cu foil, an organic substrate or a ceramic substrate. Surface mount modules or packages may be connected to metal pads on the surface of the circuit board by screening a paste containing solder particles (solder paste) onto the pads, placing the component on the circuit board surface with surface mount leads on the paste, heating the circuit board to melt the metal particles to form molten solder connecting between the leads and pads.
Testing of integrated circuits is one of the most critical steps in the manufacture of computers. If the circuits are not thoroughly tested then defective circuits could be used to make expensive products which may subsequently fail. Some defects may even lead to failure of other components of computer systems. Thus testing is one of the most important steps in the production of integrated circuits. In early computer systems, the testing of circuits that included only discrete components was simpler because the leads of each component were available for testing. In integrated circuits the semiconductor devices are not directly accessible. The devices are covered with multiple wiring layers and multiple dielectric layers and the wiring layers are covered with passivation. Thus, only a relatively small number of leads may be accessible for testing millions of devices. Complex integrated circuits sometimes include special circuits to facilitate testing other portions of the integrated circuit.
In critical computer systems integrated circuits are exposed to functional testing which may include burn-in to eliminate any early failure type defects. The functional testing is intended to simulate field operation conditions, but for complex circuits it is not possible to simulate every potential operating condition.
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